Atomization of liquids

ABSTRACT

Electrostatic spraying of liquid compositions is carried out by supplying the liquid to a spray orifice, preferably of capillary dimensions, having a charged surface which is electrically conducting or semi-conducting and which is adjacent a field intensifying electrode, the arrangement being such that the liquid is drawn out primarily by electrostatic forces, atomized into electrically charged particles without substantial corona discharge and projected past the field intensifying electrode. The spraying process and apparatus has particular utility in the spraying of liquid pesticide compositions at ultra-low volume, in that the droplets are of uniform size and are electrostatically attracted to the plants so as to wrap around the leaves of the planets and coat both the upper and lower surfaces of the leaves.

This invention relates to the spraying of pesticides, and to apparatustherefor. It is a continuation-in-part of my U.S. application Ser. No.812,440, filed July 1, 1977 now abandoned.

The application of liquid formulations to crops by means of sprayers hasbeen practised for many years, and a wide variety of machines has beenused or proposed for the purpose. More recently it has become apparentthat a crucial feature of pesticide sprayers is the particle size of thespray droplets they produce. Many simple pressure nozzle designs producea broad spectrum of spray droplet sizes, e.g. from about 10 microns toabout 500 microns in diameter. Of these, the larger droplets are lesseffective in biological action, while the smaller droplets are prone todrift in air currents, and may thus be carried away from the target forwhich they are intended (which is wasteful) to other parts of theenvironment (where they may cause actual harm). Different sizes of sprayparticle are required for different purposes. Thus, the ideal size ofspray particle of a particular insecticide, for use against a particularinsect pest in a particular crop, may be as low as 30 microns. For aherbicide, in particular circumstances, the ideal droplet size may be200 microns. Sprayers which can be set to reliably produce spraydroplets of each of these sizes (or other desired sizes in between) withfew droplets either substantially larger or smaller, have been soughtfor some time.

Most pesticides are applied to crops as water-based sprays. The activeingredient, in the form of a concentrated formulation which may beeither solid or liquid, is added to a spray-tank containing water(typically carried on a tractor), the contents being thoroughly mixed,and applied to crops. The water-based spray is normally very dilute. Atypical rate of pesticide application would be 1 kilogram of activeingredient per hectare, while a typical rate of application of dilutedspray might be 200-500 liters per hectare. Thus the concentration ofpesticide in the spray may be of the order of 0.5% or less.

This method of application thus involves the unnecessary transport oflarge volumes of water. On a large scale, a tractor is required to carrythe water around. In some terrains (e.g. steep hillsides) tractorscannot operate; and in some countries water is not readily available inlarge quanities. For this and other reasons, systems have been developedfor spraying using much lower volumes of diluent. So-called `ULV`(ultra-low volume) techniques use a spray application rate of from about0.1 to about 5 liters per hectare. Typically oils are used as the spraydiluent. The sprays are obviously much more concentrated than thehigh-volume aqueous sprays and so much less weight of spray is requiredper unit area. Thus ULV techniques are much better suited to sprayingsystems carried by men, or by aircraft, than are more conventionaltechniques. They can also be safer and more reliable, as pesticideformulations can be supplied ready for use, eliminating operator errorsin dilution and reducing opportunities for spillage and poisoningaccidents.

In recent years, substantial advances have been made in pesticidesprayers that will give spray droplets of consistent and controllablesize, particularly for ULV spraying. Sprayers have been produced inwhich the liquid pesticide is atomised by being flung off a spinningdisc or cup. In such sprayers, the droplet size depends, for a givendisc diameter, on the speed of rotation of the disc; and the sizedistribution is remarkably narrow. Hand-held ULV sprayers using thisprinciple have commanded a wide market in several parts of the world,and in particular in Africa for insecticide treatment of cotton.Nevertheless they have some drawbacks. The rotating discs are driven byelectric motors powered by dry cells, and these have to be replaced atquite frequent intervals. Also a hand-held device of this kind,containing a motor and other moving parts, is inherently liable tooccasional defects, e.g. through misuse.

Paint-spraying has for many years been carried out by means of sprayerswhich impart an electrostatic charge to the paint spray particles.Electrostatic paint-spraying has a number of advantages. The chargedparticles are attracted to the object being sprayed, and thus less arewasted; the electrostatic field promotes adhesion and even coating;moreover the field carries particles round behind the object, to coatthe back of it. Apparatus used in electrostatic paint-spraying has beenof two types, both associated with the name of Ransburg. In the earliestapparatus, paint was atomised from a conventional nozzle, and passedthrough a wire grid mesh, held at a potential of the order of 100,000volts. This charged the paint spray by ionic bombardment. In asubsequent development, paint was supplied to a rotating disc, from theedges of which it atomised as charged particles under the influence of ahigh potential (usually about 70,000-80,000 volts) applied to the disc.Other devices have been proposed in which the paint is atomised by airblast, before or during charging. First introduced in the 1940's,electrostatic paint-spraying machines are in widespread use today,typically for such applications as painting automobile parts. Theyrequire considerable amounts of electric power, usually provided frommains electrical supplies, and are not readily portable.

It has been recognised for many years that electrostatic spraying ofpesticides could have advantages corresponding to those obtainable byelectrostatic paint spraying, i.e. more even coating of plants,including coating of the backs of leaves, better adhesion to plants, andless pesticide wasted (in particular, by loss of pesticide to thesurrounding environment, where it may do harm). A number of properproposals for applying electrostatically charged pesticides (usually asparticulate solids) have been made, but to the best of Applicant'sknowledge no such prior proposals have yet come into substantialcommercial use anywhere in the world. Difficulties have arisen in partfrom the need to use high potentials to charge the pesticide sprays,which in turn have required expensive and bulky apparatus, which in somecases has been too heavy even to be conveniently carried on a tractor.

Objects of the present invention include the following:

to provide a practical system for the electrostatic spraying of liquidpesticides;

to provide an improved process for ULV spraying of pesticides, in whichparticle size may be readily controlled, using apparatus which issimple, reliable, light enough to be held in the hand or readily carriedby aircraft, and has a much reduced power requirement compared withknown ULV spraying devices;

to provide a system wherein pesticides may be sprayed on to plants withbetter and more even coverage of foliage, and less loss of pesticide tothe environment, offering possibilities of saving in pesticide usage.

These and other objects may be attained by the following invention,which consists in spraying apparatus for use in the electrostaticspraying of pesticides which comprise a spray-head having an at leastsemi-conducting surface; means for electrically charging the spray-headto a potential of the order of 1-20 kilovolts; means for deliveringpesticide spray liquid to the surface; a field intensifying electrodemounted adjacent to the surface, and means for connecting the fieldintensifying electrode to earth; the electrode being so sited relativeto the surface that when the surface is charged, the electrostatic fieldthereat causes liquid thereon to atomise without substantial coronadischarge to form electrically charged particles which are projectedaway from the electrode.

The invention further consists in a process of spraying liquidpesticides at ultra-low volume, which comprises supplying a relativelyconcentrated composition of a pesticide in an organic diluent to anelectrically conducting surface adjacent a field intensifying electrode,the electrode being at such a potential and so sited relative to thesurface that an atomising field strength is created at the surface sothat the liquid is atomised at least preponderantly by electrostaticforces substantially without corona discharge to form electricallycharged particles which are projected away from the electrode.

When a liquid is displaced from the locality of an electricallyconducting surface at a voltage above or below earth potential theliquid may upon emerging into free space carry a net electrical chargeresulting from an exchange of electrical charges with the source of theelectrical potential. In my invention, this technique is used to atomisethe displaced liquid since the net electric charge in the liquid as theliquid emerges into free space from the locality of the conductingsurface counteracts the surface tension forces of the liquid. The amountof electrical charge in the emerging liquid droplets after atomisationis, in part, dependent upon the strength of the electric field at theconducting surface.

A salient feature of known paint-spraying devices is that a combinationof high voltage and sharp-edged conducting surfaces causes breakdown ofthe surrounding air (by the phenomenon known as corona discharge). Theeffect of this is that not all of the current supplied to the conductingsurface is used to charge the liquid. Thus, corona discharge results inunnecessary current loss and greatly increases the current drawn fromthe source of high electrical potential. This has disadvantages. Oneserious disadvantage is that the power required of the high electricalpotential source is too high to be met easily by portable energy sourcese.g. torch batteries.

In the present invention an electrode hereinafter referred to as a fieldintensifying electrode, is placed in close proximity to the conductingsurface and enables a sufficiently high field strength to be created atthe conducting surface using a relatively low voltage, of the order of1-20 KV, to charge the droplets. Thus a high charge density for example,of the order of 10⁻² coulombs/kilogram may be placed upon the liquid.This gives rise to a high charge-utilisation efficiency which in turnenables low power sources, such as piezo-electric crystals, torchbatteries or solar cells to be utilised as a charge transfer device, andto give rise to electrostatic atomisation of the liquid.

Such atomisation requires no mechanical assistance such as an air blastor rotating disc. The combined field due to the voltage on theconducting surface plus the space charge of the atomised liquid itselfthen enables the droplets to be targeted toward an earthed object, or toform an airborne (aerosol) cloud.

The field intensifying electrode may be considered to be a `dummytarget` since it strongly influences the field in the region of liquidatomisation. But, unlike an actual target, it is placed close to theconducting surface thus strengthening the field. We have found that thefield adjusting member may easily be placed so that it does not itselfbecome a target for the atomised spray.

The reason for this is not fully understood, but observation shows that,provided the liquid's physical characteristics (e.g. resistivity,viscosity) and flow rate are such as to cause the liquid to leave theconducting surface under the action of the electrostatic field in theform of ligaments of about 1 cm or more, the atomisation will take placein that part of the electric field where the combined forces of inertia,gravity field, and electrostatic field are directed away from the fieldintensifying electrode.

It has been found possible to cause impingement of the spray on to thefield intensifying electrode by placing it downstream of the atomisingtip of the ligament. In this case it has been noticed that, withrelatively small amounts of impinging liquid, that provided the surfaceof the field intensifying electrode is sufficiently conducting, andearthed, the impinging particles give up their charge and take up anopposite charge by induction in the electric field. This causes them tore-atomise and not to be retained on the electrode. Moreover, any verysmall droplets which may be formed in the atomisation process arepreferentially captured by the electrode because of their low momentum.

By the term `conducting surface` we mean the surface of a materialhaving a resistivity of the order of 1 ohm cm or less, and by`semi-conducting surface` we mean the surface material having aresistivity value of between 1 and about 10.sup. ohm cm. By `insulatingmaterial` is meant material having a resistivity of more than 10¹² ohmcm.

The conducting or semi-conducting surface adjacent which the liquidatomises may have various shapes. It will often be the end of a sprayconduit, preferably a conduit of capillary size, for example, a nozzleaperture, through which in operation the liquid spray emerges.

The conducting surface may also comprise the edges of two concentrictubes which edges define an annular aperture through which liquidemerges. The edges of the tubes may be serrated or fluted.Alternatively, the conducting surface may comprise two edges defining aslot, preferably of capillary width. The slot may be of rectangular orother form. Atomisation may be effected from the flat surface of a solidconductor or semiconductor to which liquid has been supplied.

The geometric shape of the field intensifying electrode in generalfollows the shape of the conducting or semi-conducting surface. Wherethe surface is defined by a nozzle the electrode may take an annularform with the member encircling the nozzle.

The field intensifying electrode is generally sited as close as possibleto the conducting surface without corona discharge occurring betweenthem. For example with 20 KV on the conducting surface the electrode ispreferably sited not less, and not much more than, about 2 cm away fromit. The electrode may be sited either level with, in front of, or behindthe conducting surface from which the liquid atomises. In one form ofthe invention the field intensifying electrode has an insulatingsurface. For example, it may be a thin wire embedded in a body or sheathformed of a plastics material. This enables the distance between theelectrode and the conducting surface to be very much smaller than wouldbe obtainable with `air-gap` insulation only. This results in anenhanced field strength in the locality of the conducting surface.

The field intensifying electrode may be adjustably mounted on theapparatus of the invention so that the spatial relationship between theelectrode and the surface can easily be varied.

We have found that the position and the geometric shape of the fieldintensifying electrode control the angle of the stream of dropletsemerging into free space. When the electrode is behind the emergingspray the angle of the stream is increased, and when it is in front ofthe emerging spray the angle is decreased.

In addition, we have found that the average size of the atomiseddroplets in general may be controlled by the position of the fieldintensifying electrode in relation to the conducting surface. Forexample, for a given flow rate of liquid, bringing the electrode closerto the conducting surface results in the droplets generally being of asmaller average size. This is because the field at the surface isintensified; a similar control of droplet size may be obtained byincreasing or decreasing the applied voltage, which causes acorresponding increase or decrease in the electrostatic field and aconsequent decrease or increase, respectively, in particle droplet size.

By controlling the electrostatic field a selected size of droplets maybe produced suitable for a particular use. For example, large numbers ofsmall particles (e.g. 20-30 μ) of an insecticide may be preferred formaximum coverage of a target, whereas for a herbicide larger dropletsless prone to wind drift may be required. This selected droplet size canbe maintained notwithstanding the movement of the target relative to theconducting surface because the field strength created between the fieldintensifying electrode and the surface outweighs that produced by thetarget.

We have found also that for a given voltage and a fixed fieldintensifying electrode position the droplet size of a given liquid isrelated to throughput.

The apparatus may also comprise one or more additional electrodes tofurther influence the spray pattern. For example, if in a systemcomprising a conducting nozzle and an earthed circular fieldintensifying electrode around it, a second earthed circular member isplaced outside the first, this will broaden the spray swath; andconversely a second earthed circular member of smaller cross-sectionalarea disposed downstream of the nozzle will narrow the spray swath.

We have found that how well a liquid is atomised depends on thepotential on the surface, the position of the field intensifyingelectrode, the liquid throughput, and the nature of the liquid. Forpractical purposes we have found that highly non-polar liquids, e.g.pure hydrocarbon solvents, and highly polar liquids, such as water, donot atomise so well. Our invention is particularly suited to theatomisation of pesticides dissolved or suspended in organic liquiddiluents, in particular formulations having a viscosity at 20° C. ofbetween 1 and 50 centipoises and a resistivity at 20° C. of between 10⁶and 10¹⁰ ohm centimeters.

Atomisation of a liquid effected by the process or apparatus accordingto the invention requires no mechanical assistance such as a forced airblast or rotating disc. However, once the liquid has been atomised andhas passed out of the atomising field a forced air blast may be used toproject the atomised droplets over greater distances to a target, thusfor example assisting penetration through foliage.

As previously discussed, the use of a rotating disc as a surface toatomise liquid pesticides is known. However, if such a surface ischarged and provided with a contiguous field intensifying electrode,atomisation and spraying trajectories are influenced by both inertialand field-effect `electrostatic` forces. Surprisingly, it is found thatboth of these forces combine favourably even at potential differences ofthe order of 10 KV or less, to produce much finer atomisation than wouldbe obtained from the inertial effect alone. For example, with air-gapinsulation only between the field intensifying electrode and theconducting surface at a potential difference of about 20 KV, using a3-inch diameter disc rotating at 1,500 revolutions per minute as theconducting surface, a droplet mean diameter of the order of 20-30μ hasbeen observed at a flow rate of 1.0 cc per second. Other combinations ofrotational speed and applied voltage give different effects. It ispossible to use relatively low voltages (of the order of 1 KV or so) andrather higher rotational speeds (say 5,000 to 8,000 rpm) to giveparticles of a useful size range.

Under certain conditions, for example if the throughput of liquid ishigh enough, a powerful space-charge may be created between the spraynozzle and its target due to the presence of large numbers of chargedparticles. This space-charge may be sufficiently large to repel veryfine charged particles emerging from the nozzle, giving them anappreciable component of velocity in a direction normal to, or evenopposite to, the nozzle-target direction. We have termed this effect`back-spray`.

A suitably placed deflector electrode at a high potential may preventthis `back-spray`.

Accordingly, in a further feature of the invention there is providedspraying apparatus comprising spraying apparatus according to theinvention as hereinbefore defined and further comprising a deflectorelectrode capable of receiving a high potential and so sited in relationto the nozzle spray that `back-spray` is prevented.

The deflector electrode may be formed of a metal such as steel oraluminium. When the field intensifying member is of an annular form thedeflector electrode may take the form of a co-axial ring of slightlygreater diameter than that of the field intensifying electrode, anddisposed slightly behind it. The deflector electrode may be mounted onan insulating support so as to be fixed in space and retain charge. Adisc formed of a plastics material such as "Perspex" may be used forthis purpose.

The voltage on the deflector electrode may be set by either:

(a) a tapping from the high-voltage source used to charge the conductingsurface of the spraying apparatus, either directly, or via a potentialdivider of very high resistance to prevent unwanted power dissipation;or,

(b) a separate source of high voltage, which could be of lower powerrating since the deflector electrode is not essentially an active devicebecause no power is consumed in its operation.

Typically, when the conducting surface has a voltage of 20 KV, asuitable voltage for the deflector electrode would be 15-20 KV. Also,typically, the total resistance of a suitable potential divider would beof the order of 10¹¹ ohms. Such a resistance can be realised by use of asemi-insulating material of about 2 cm length and of 1 square cmcross-section (any geometric shape) having electrodes placed at the endsof the material, and together with a tapping electrode suitably setbetween the ends to obtain the potential division required. Strips ofwood, cardboard, and rubber-like materials may be used.

In yet a further feature of the invention there is provided sprayingapparatus which comprises two or more spraying devices according to theinvention mounted on a boom. The boom may be hand-held, or mounted on,or comprising part of, a tractor or aircraft. Such devices according tothe invention are of particular use in multi-row crop spraying, and forthe spraying of crops and weeds by aircraft mounted sprayer.

Some embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is an elevational view, schematically illustrating the principalcomponents, of a preferred electrostatic spray gun according to theinvention;

FIG. 2 is a cross-sectional view of the gun nozzle as shown in FIG. 1;

FIG. 3 is an underside view of the gun nozzle of FIG. 2;

FIG. 4 is an electrical circuit diagram of the spray gun of FIG. 1;

FIG. 5 is an elevational view, part cut away, schematically illustratingthe principal components of a spray pistol according to the invention;

FIG. 6 is an electrical circuit diagram of the spray pistol of FIG. 5;

FIG. 7 is a cross-sectional view of a gun nozzle comprising twoconcentric tubes for a spray gun according to the invention;

FIG. 8 is an underside view of the gun nozzle of FIG. 7;

FIG. 9 is a cross-sectional view of a gun nozzle comprising a solidconducting block for a spray gun according to the invention;

FIG. 10 shows the spray gun of FIG. 1 further comprising a deflectorelectrode;

FIG. 11 is a cross-sectional view of the gun nozzle shown in FIG. 10;

FIG. 12 is a perspective view of a head of a spraying apparatusaccording to the invention comprising a linear slit arrangement;

FIG. 13 is a cross-sectional view on the line I--I of FIG. 10;

FIG. 14 is an underside view in part of the apparatus of FIG. 10.

Referring to FIG. 1, the electrostatic spray gun comprises a hollow tube1 formed of a plastics material and providing a firm holding support forother parts of the gun. Within the tube 1 is a bank of sixteen 11/2 voltbatteries 2 which acts as the electrical energy source. Attached to theside of the tube 1 is a Brandenburg 223P (0-20 KV, 200 microamp) highvoltage module 3 connected to the batteries 2 and to a `ON-OFF` switch4, and providing a source of high electrical potential. The tube 1 atits forward end has an integral, internally screw-threaded eye 5 adaptedto receive a bottle 6 containing liquid to be sprayed. The eye 5 at itslower part holds the upper part of a tubular distributor 7 formed of aninsulating plastics material and supporting in its lower end a disc 8(FIG. 2) of the same material. Now, referring more specifically to FIG.2, projecting through the disc 8 are eight metal capillary tubes 9 whichform the spray nozzle assembly. The capillary tubes 9 are each solderedto a bare-metal wire 10 which in turn is connected to the high potentialterminal of the module 3 via a high potential cable 11.

Encircling the distributor 7 is an inverted dish 12 formed of aninsulating plastics material. Supported in the lip of the dish 12 is ametal field intensifying ring electrode 13 electrically connected toearth by an earth lead 14. The dish 12 may be moved up and down thedistributor 7 but fits sufficiently closely thereon to maintain byfrictional engagement any position selected.

To assemble the spray gun for use, the bottle 6, containing liquid to besprayed, is screwed into the eye 5 while the spray gun is inverted fromthe position shown in FIG. 1. Inverting the spray gun back to theposition shown in FIG. 1 allows the liquid to enter the distributor 7and to drip out of the capillary tubes 9 under gravity flow.

In operation to spray liquid, the spray gun is held by hand at asuitable position along the length of the tube 1.

On turning switch 4 to its `ON` position, the capillary tubes 9 becomeelectrically charged to the same polarity and potential as the outputgenerated by the module 3. This results in the liquid emerging from thetubes in an atomised and electrostatically charged form when the gun isinverted to the spraying position.

When the field intensifying electrode 13 is earthed, via earth lead 14,the electrostatic field at and around the capillary tubes 9 improvesboth the atomisation and the spray pattern even when the potential onthe spray nozzle assembly is at only, say, 10 to 15 kilovolts (eitherpositive or negative polarity with respect to the field intensifyingelectrode 13). Furthermore, due to the close proximity of the fieldintensifying electrode 13 to the spray nozzle assembly, the currentdrawn from the source of high potential 3 is mainly that which arisesfrom an exchange of charge between the capillary tubes 9 and the liquidbeing sprayed, and is thus extremely small.

Typically, the charge density of the atomised liquid is 1×10⁻³ coulombper liter. Thus, at a liquid flow rate of, say 1×10⁻³ liter per secondthe current drawn from the module 3 is only 1×10⁻⁶ ampere, indicating anoutput power of only 1×10⁻³ watt when the high potential is 1×10³ volts.At this low power, the useful life of the batteries 2 used to energisethe module 3 may be hundreds of hours. Even if the charge density israised somewhat (e.g. to 10⁻² coulombs per liter) the power consumptioncan still be very low (e.g. 200 milliwatts).

To maintain the electrode 13 at low or zero potential, the earth lead 14must contact actual ground or some other low voltage, high capacitance,body. For portable use of the spray gun shown in FIG. 1, it issufficient to trail the earth lead 14 so that it touches or occasionallytouches the ground. The spray gun may be used for short periods of timewithout the earth lead 14 being connected to earth, without noticeablyaffecting the spray characteristics. Even when the earth lead 14 is notelectrically earthed at all the spray gun will continue to sprayelectrostatically, albeit with a deterioration in performance.

By varying the position of the dish 12 along the length of thedistributor 7 the position of the electrode 13 may be adjusted withrespect to the fixed position of the capillary tubes 9 so as to achievethe best spray characteristic in accordance with the potentialdifference between the electrode 13 and the capillary tubes 9, and othervariables such as the electrical resistivity of the liquid.

The specific embodiment described hereinabove was tested as describedbelow.

In a first test conducted outdoors a liquid insecticide formulation(resistivity approximately 5×10⁸ ohm cm) was electrostatically sprayedagainst a set of earthed vertically placed metal tubes, each of 1 inchdiameter, placed in a downward line at distances of 1 to 15 meters fromthe spray gun; the liquid being atomised at a height of about 1 meterabove the ground. A comparative test was conducted using a commerciallyavailable mechanical atomising device used for agricultural sprayingwherein atomisation is produced from an uncharged spinning disc.

It was found that the droplets from the electrostatic spray gun weredeposited more uniformly on all of the metal tubes than those from themechanical atomiser. The electrostatic spray gun clearly demonstrated asignificant `wrap-round` effect.

In a second test, the first test was repeated but with the 223P; 0-20KV; 200 microamp module 3 (ex. Brandenburg Ltd) being replaced with a 11KV unit having no regulation or feedback control and being capable ofdelivering an output of only 1 microamp at about 11 KV.

In this test the liquid was electrostatically atomised and sprayedsatisfactorily.

The apparatus shown in FIG. 1 may be used to produce anelectrostatically charged pesticide aerosol cloud, i.e. a cloud ofdroplets having a mean droplet size of less than 50 microns in diameterand generally in the range of 1-10 microns. The apparatus of FIG. 1having capillary tubes with an internal diameter of 0.1 mm, and using aliquid having a resistivity approximately 5×10⁸ ohm cms at a total flowrate of 0.05 cc/second per eight capillary tubes produces such anaerosol cloud.

A further embodiment of the invention is the electrostatic spray handpistol shown in FIG. 5. In this embodiment the source of high potentialcomprises lead zirconate crystals which generate the potential by meansof the well-known `piezoelectric effect`.

The hand piston shown in FIG. 5 comprises a pistol-shaped casing 21formed of an insulating plastics material, and a metal trigger 22 (shownin FIG. 5 in a released position). The upper part of the trigger 22 isshaped to form a cam 23.

Within the handle of the pistol are two lead zirconate crystals 24 (typePZT4, manufactured by Vernitron Ltd, Southampton, England) having acentre tap connection 25. The crystals 24 each have an upper face 26which in operation is acted upon by the cam 23.

Fitted to the end of the nozzle of the pistol is a distributor 27 formedof an insulating plastics material which holds at its end adjacent thenozzle a disc 28 formed of the same material. Protruding through thedisc 28 into the distributor 27 is a feed tube 29, having a tap 30,which is connected to a feed bottle 31 which holds the liquid to besprayed.

The distributor 27 at its other end has a disc 32 formed of aninsulating plastics material through which protrude eight metalcapillary tubes 33 which form the spray assembly. The capillary tubes 33are each soldered to a bare-metal wire 34 which in turn is connected tothe centre tap connection 25 via a high potential cable 35 providedwithin the barrel of the pistol.

Encircling the distributor 27 is a cylindrical support 36 formed of aninsulating plastics material. The support 36 may be moved along thelength of the distributor 27 but fits sufficiently closely thereon tomaintain by frictional engagement any position selected. Embedded in thesupport 36 is a metal field intensifying ring electrode 37 which iselectrically connected to the trigger by an earth lead 38.

In operation to spray liquid, the tap 30 is turned on. This allowsliquid to flow under gravity from the feed bottle 31 along the feed tube29 into the distributor 27 and to emerge dropwise out of the capillarytubes 33.

On squeezing the trigger, the cam 23 acts on the faces 26. This actioncompresses the crystals 24 and results in the generation of a potentialdifference, which is transmitted via the cable 35 to the capillary tubes33. This results in the liquid emerging from the tubes 33 in an atomisedand electrostatically charged form.

When the electrode 37 is earthed, via earth lead 38, trigger, andoperator, the electrostatic field at and around the capillary tubes 33improves both the atomisation and the spray pattern.

By varying the position of the support 36 along the length of thedistributor 27 the position of the electrode 37 may be adjusted withrespect to the fixed position of the capillary tubes 33 so as to achievethe best spray characteristic in accordance with the potentialdifference between the electrode 37 and the capillary tubes 33, andother variables such as the electrical resistivity of the liquid.

Typically, the crystals 24, when squeezed slowly for five seconds or so,produce a potential difference of about 10 KV, and have sufficientelectrical capacitance to impart at least one microcoulomb to the liquidbeing atomised during a five second squeeze. If the liquid output rateis about 1×10⁻⁴ liter per second the charge density of the atomiseddroplets is of the order of 2×10⁻³ coulombs per liter.

In a spray test using this specific embodiment the resultant sprayexhibited satisfactory atomisation and `wrap-round` when a target tubewas earthed and held at a distance of about 0.5 meter.

The pistol illustrated may readily be modified by means of a mechanicalconnection between the trigger 22 and a valve in the feed tube 29, soarranged that pressure of the trigger opens the valve and release closesit. In this way liquid only passes through the nozzles 33 when they arecharged.

Alternative gun nozzles which may be substituted for the nozzle of FIG.2 in the gun of FIG. 1 are shown in FIGS. 7-9.

The nozzle shown in FIGS. 7 and 8 comprises a hollow steel cylinder 39having a uniform bore and a lower half of reduced external diameter. Thecylinder 39 at its upper part is held by frictional engagement withinthe tubular distributor 7 of FIG. 1 and connected via the metal wire 10and cable 11 to the high potential terminal of the module 3. At itslower part cylinder 39 is closed by seal 40 and has four holes 41 ofcapillary size extending radially of the cylinder wall.

An outer steel cylinder 42 at its upper part embraces an intermediatepart of the cylinder 39 and is held by frictional engagement thereon. Atits lower part cylinder 42 defines, with the lower part of cylinder 39,an annular cavity 43. The holes 41 connect the cavity 43 with the insideof cylinder 39.

Encircling the distributor 7 is the dish 12 supporting the fieldintensifying ring electrode 13.

In use, turning the switch 4 to its `ON` position, cylinders 39 and 42become electrically charged. Liquid passing through distributor 7 passesout of holes 41 into cavity 43 and emerges therefrom in an atomised andelectrostatically charged form.

The nozzle shown in FIG. 9 comprises a solid steel cylinder 44 held atits upper part by frictional engagement with the distributor 7 ofFIG. 1. The cylinder 44 has a central axial bore 45 running almost thelength of the cylinder and terminating at a transverse bore 46 in thelower part of the cylinder. The cylinder 44 is connected to the module 3via the metal wire 10 and cable 11. The lower part of the cylinderterminates as a solid disc 47 having a bottom surface 48.

In use, when cylinder 44 becomes electrically charged, liquid fromdistributor 7 passes through bores 45 and 46 and flows around disc 47 tosurface 48 from which it is atomised.

If the flow rate of liquid out of bore 46 is sufficiently reducedatomisation of the liquid may occur from the surfaces adjacent the twoexits of bore 46.

The embodiment shown in FIGS. 10 and 11 comprises the spray gun of FIG.1 fitted with a deflector electrode system to prevent `back-spray`.

As shown in FIGS. 10 and 11 a disc 51 formed of an insulating materialembraces the distributor 7 at its mid-section and is held thereon byfrictional engagement. Partly embedded in the lower surface of the disc51 is a deflector electrode 52 in the form of a steel ring. Thedeflector electrode 52 is connected, via a high voltage cable 53, to atapping 54 of a potential divider 55. The divider 55 comprises aresistor of 10¹⁰ ohms, connected at one end to the high potential cable11 and at its other end to the earth lead 14. The high resistance ofdivider 55 minimises current drain from the high voltage source 3, andserves as a current limiter in the event of a short circuit occurring atthe deflector electrode 52.

In operation, with switch 4 in the `ON` position the deflector electrode52 receives a high potential from the potential divider 55. Suitableadjustment of the tapping 54 may give any desired potential between zerovolts and the potential of the source 3. A typical voltage on thedeflector electrode 52 would be 14 KV.

The position of the deflector electrode 52 in relation to the electrode13 and the spray nozzles 9 may be selected by moving the disc 51 alongthe length of the distributor 7.

Liquid emerging from the nozzles 9 is atomised and directed by thecombined electric field forces set up not only by the high voltage onthe nozzles 9 and the local low potential of the electrode 13 but alsoby the high potential on the deflector electrode 52.

Referring to FIGS. 12-14, the head of the spraying apparatus comprises arectangular body 61 formed of an insulating plastics material and havinga rectangular chamber 62. Along the length of its lower face, the body61 has an integrally formed upstanding projection 63 having alongitudinal slit 64 which connects with chamber 62. The upper face ofthe body 61 has an aperture 65, adapted to receive (by means not shown)a liquid to be sprayed, and which communicates with chamber 62.

The slit 64 is divided by a conducting surface formed of a thin metalsheet 66 connected to a source of high potential (not shown). Held bysupports 67 adjacent the projection 63 is an earthed metal wire 68enclosed in a sheath 69 formed of an insulating plastics material.

In operation with the high potential applied to the metal sheet 66,liquid to be sprayed enters the chamber 62, via the aperture 65. Itemerges from the slit 64 where it is atomised adjacent the metal sheet66. The wire 68 acts as a field intensifying electrode on both sides ofthe metal sheet 66. Because it has an insulated protective surface themetal wire 68 can be disposed closer to the metal sheath 66 than if itwere not so insulated, and also with a greatly reduced risk of arcing.

In an alternative embodiment the conducting surface may comprise a metalwire.

In a further embodiment utilising the linear slit arrangement amultiplicity of wire or metal sheet conducting surface in parallel anddisposed between a multiplicity of such sheathed wire field intensifyingelectrodes is used. Such an arrangement allows of an increase in thevolume of liquid to be sprayed.

The various devices described are particularly useful in the process ofthe invention, that is to say, in spraying liquid pesticides atultra-low volume. They may easily be made portable and self-contained,being conveniently powered by low output power sources such as drycells, piezoelectric sources or photoelectric sources. The process ofthe invention has particular advantages over known methods of sprayingliquid pesticides because it can give a more even coating of pesticideson foliage. Electrostatic forces direct the spray particle to theirtarget, reducing drift, and enable leaves to be coated on both sides.Liquid pesticidal compositions sprayed by the process of the inventionmay be for example insecticides, fungicides and herbicides. Typicallythey are in the form of solutions or dispersions of a pesticide in apesticidally inert organic diluent (e.g. a liquid hydrocarbon) but it isalso possible to spray liquid pesticides substantially undiluted.Because deposition is uniform, drift is minimised, and low flow-ratescan be used, the apparatus is particularly suitable for applyingpesticides undiluted or in highly concentrated formulations (ultra-lowvolume spraying).

What I claim is:
 1. A process of spraying pesticides at ultra-low volume onto plants comprising supplying a liquid pesticidal composition from a source thereof through a liquid passage to a spray orifice which has an at least electrically semiconducting surface adjacent a field intensifying electrode, said surface being electrically charged and the electrode being at such a potential and so sited relative to said surface that an atomising field strength is created at said orifice so that the liquid pesticide at said orifice is drawn out preponderantly by electrostatic forces substantially without corona discharge into ligaments which break up into electrically charged particles which are projected away from the electrode and into contact with the plants.
 2. A process of spraying pesticides as claimed in claim 1 in which the size of the particles is controlled by control of the field strength at the surface.
 3. A process of spraying pesticides as claimed in claim 2 in which the field strength is controlled by varying the distance of the field intensifying electrode from the surface.
 4. A process as claimed in claim 1 in which the field intensifying electrode is at earth potential.
 5. A process as claimed in claim 1 in which the liquid pesticidal composition is a solution or dispersion of a pesticide in a pesticidally inert organic diluent.
 6. An electrostatic spraying process as in claim 1 wherein the spray orifice is of capillary dimensions.
 7. An electrostatic spraying process as in claim 1 wherein the liquid pesticidal composition has a resistivity in the range of about 10⁷ to 10⁸ ohm cms.
 8. An electrostatic spraying process as in claim 1 wherein said surface is charged to a potential of the order of 1-20 kilovolts.
 9. An electrostatic spraying process as in any one of claims 6, 7 or 8 wherein the potential applied to said surface is a single polarity potential.
 10. An electrostatic spraying apparatus suitable for spraying pesticides at ultra-low volume into contact with a plant, comprising:a spray-head having a spray orifice which has an at least electrically semi-conducting surface; means for electrically charging said surface to a potential of the order of 1-20 kilovolts; means for delivering spray liquid through said spray orifice; a field intensifying electrode mounted adjacent to said surface; and means for connecting the field intensifying electrode to earth; the electrode being so sited relative to said surface that when said surface is charged, the electrostatic field thereat causes liquid at said orifice to be drawn out preponderantly by electrostatic forces without substantial corona discharge into ligaments which break up into electrically charged particles which are projected past the electrode and into contact with the plant.
 11. Apparatus as claimed in claim 10 in which the field intensifying electrode is sited forward of said surface.
 12. Apparatus as claimed in claim 10 in which the field intensifying electrode is adjustably mounted on the apparatus so that the distance between the electrode and said surface can be varied, thereby varying the field strength at the surface.
 13. Apparatus as claimed in claim 10 in which the orifice is of capillary dimensions.
 14. Apparatus as claimed in claim 10 in which the field intensifying electrode is sited as close as possible to said surface without discharge occurring between them.
 15. Apparatus as claimed in claim 10 in which the field intensifying electrode is covered with an insulating material.
 16. Apparatus as claimed in claim 10 in which the field intensifying electrode is sited level with said surface.
 17. Apparatus as claimed in claim 10 which further comprises a deflector electrode operably capable of maintaining a high potential of the same sign as the atomised liquid, and so sited between the field intensifying electrode and the body of the apparatus as to prevent `back spray`.
 18. A portable, self-contained electrostatic spray gun suitable for use in spraying pesticides at ultra-low volume into contact with a plant comprising:a reservoir for containing liquid to be sprayed; a spray-head having a spray orifice which has an at least electrically semi-conducting surface adjacent which liquid may atomise; means for delivering the liquid from the reservoir through said spray orifice; a field intensifying electrode in close proximity to said surface; means for connecting the field intensifying electrode to earth; and a power source adapted to charge said surface to a potential of the order of 1-20 kilovolts; the electrode being so sited relative to said surface that when said surface is charged, the electrostatic field thereat causes liquid at said orifice to atomise without substantial corona discharge to form a cloud of electrically charged particles which are projected past the electrode.
 19. An electrostatic spray gun as claimed in claim 18 wherein the power source is one or more dry cells.
 20. An electrostatic spray gun is claimed in claim 18 wherein the power source includes a piezoelectric element.
 21. An electrostatic spray gun as claimed in claim 18 wherein the power source includes a photoelectric element.
 22. An electrostatic spray gun as claimed in claim 18 further comprising means for varying the electrostatic field at said surface thereby to determine and control the droplet size of the atomised liquid.
 23. An electrostatic spray gun as in claim 18 wherein the spray orifice is of capillary dimensions.
 24. An electrostatic spray gun as in claim 18 or claim 23 wherein the potential applied to said surface is a single polarity potential.
 25. A process for spraying pesticides at ultra low volume on to plants growing in the earth comprising the steps of:supplying an organic pesticidal liquid composition from a source thereof through a liquid passage under gentle hydrostatic pressure within a predetermined range of pressures to a spray orifice in a nozzle, the orifice having an at least electrically semi-conducting surface, where it is adjacent and spaced from an electrically conducting electrode; applying an electrical potential to said surface; providing an electrical pathway connecting said electrode to earth, the magnitude of the potential and the position of the electrode being such that the liquid at said surface is drawn out preponderately by electrostatic forces without corona discharge into ligaments which break up into electrically charged droplets of uniform size which are projected towards the plants; and moving said source, said nozzle and said electrode past the plants.
 26. An electrostatic spraying process as in claim 25 wherein the spray orifice is of capillary dimensions.
 27. An electrostatic spraying process as in claim 25 wherein said orifice is of capillary dimensions.
 28. An electrostatic spraying process as in claim 25 wherein said surface is charged to a potential of the order of 1-20 kilovolts.
 29. An electrostatic spraying process as in any one of claims 26, 27 or 28 wherein the potential applied to said surface is a single polarity potential.
 30. An electrostatic spraying apparatus suitable for spraying pesticides at ultra-low volume into contact with a plant, comprising:a spray head having at least one annular spray orifice of capillary dimensions defined by the edges of two concentric tubes, at least part of the surface of the edges being at least electrically conducting; means for electrically charging said surface to a potential of the order of 1-20 kilovolts; means for delivering spray through said orifice to said surface; a field intensifying electrode mounted adjacent to said surface; and means for connecting the field intensifying electrode to earth; the electrode being so sited relative to said surface that when said surface is charged, the electrostatic field thereat causes liquid thereon to atomise without substantial corona discharge to form electrically charged particles which are projected past the electrode and into contact with the plant.
 31. An electrostatic spraying apparatus suitable for spraying pesticides at ultra-low volume into contact with a plant, comprising:a spray-head having at least one slot-shaped spray orifice of capillary dimensions defined by the edges of two substantially parallel edges, at least part of the surface of the edges being at least electrically conducting; a spray-head having an at least electrically semi-conducting surface; means for electrically charging the spray-head surface to a potential of the order of 1-20 kilovolts; means for delivering spray liquid to the surface; a field intensifying electrode mounted adjacent to the spray-head surface; and means for connecting the field intensifying electrode to earth; the electrode being so sited relative to the spray-head surface that when the spray-head surface is charged, the electrostatic field thereat causes liquid thereon to atomise without substantial corona discharge to form electrically charged particles which are projected past the electrode and into contact with the plant.
 32. Electrostatic spraying apparatus for spraying liquids having a resistivity in the range of about 10⁷ to 10⁸ ohm cms comprising:a liquid reservoir; a spray-head for said liquid; the spray-head including an inner cylinder and an outer tube coaxial therewith defining a liquid passage and an annular spray opening, at least one of said tube or cylinder having an at least electrically semiconducting surface forming a boundry of said spray opening; means for delivering liquid through said liquid passage to the spray-head from the reservoir under a gentle hydrostatic pressure within a predetermined range of pressures sufficient to cause flow through said spray opening; an annular field adjusting electrode mounted coaxially of said spray opening and means for electrically connecting said annular electrode to earth; and means for charging said at least electrically semiconducting surface to create an electrostatic field between said spray-head and said annular electrode sufficient without corona discharge to draw liquid delivered to the said spray opening out into ligaments which break up into electrically charged droplets of uniform size that are projected outwardly from the spray-head.
 33. Electrostatic spraying apparatus as in claim 32 wherein the spray opening is of capillary dimensions.
 34. Electrostatic spraying apparatus as in claim 32 or 33 wherein the charge applied to said surface is a single polarity charge.
 35. A method of treating plants growing in the ground with a liquid pesticide at ultra low volume comprising: providing a source of concentrated pesticide in a liquid organic diluent having an electrical resistance of about 10⁸ ohm cm, supplying a low-volume stream of liquid from the source to a nozzle orifice having an exterior surface of electrical resistivity up to about 10¹² ohm cm, said orifice being adjacent and spaced from an electrically conducting field-adjusting electrode; applying an electrical potential of the order of 20 kilovolts of a single polarity to said nozzle orifice surface; providing an electrical pathway connecting the electrode to the ground in which the plants are growing, the magnitude of said electrical potential and the electrode being so sited relative to said nozzle surface that when said nozzle surface is charged an electrostatic field is created thereat sufficient without substantial corona discharge to draw out the liquid pesticide from said nozzle surface in the form of liquid ligaments of 1 cm or more in length and to cause the ligaments to break up into a cloud of charged atomized droplets which is projected away from the electrode, the cloud being electrostatically attracted to the plants so as to wrap around leaves of the plants and coat both the upper and lower surfaces of the leaves; and moving the source, the nozzle orifice and the electrode past the plants.
 36. A method as in claim 35 wherein said electrical pathway between the field-adjusting electrode and the ground is provided by a flexible electrical conductor which contacts the ground and which is moved along the ground as the source, the nozzle and the electrode move past the plants.
 37. A method of treating plants as in claim 35 wherein the nozzle orifice is of capillary dimensions.
 38. A method of treating plants as in claim 35 or 37 wherein the potential applied to the nozzle orifice surface is a single polarity potential.
 39. A portable electrostatic sprayer suitable for hand-held electrostatic spraying of liquid pesticide formulation onto useful or unwanted target plants, comprising: an elongate body serving as a handle for said sprayer; an electrically conductive annular nozzle supported at one end of said body for downwardly directed spraying and defining an annular spray orifice; a bottle mounted on said body for delivering liquid pesticide formulation by gravity flow from the bottle to said nozzle for atomisation therefrom; an electrical power source mounted on said body; a high voltage generator for activation by said power source and for connection to said nozzle to charge it to a potential of the order of 20 kilovolts; an annular electrode mounted co-axially with said nozzle parallel to and above the plane of said orifice; and an earth lead connected to said annular electrode for at least intermittently connecting said electrode to earth to maintain the same at or near earth potential; the annular electrode being so sited relative to the orifice that when the nozzle is charged by the generator the electrostatic field at said orifice is sufficient without substantial corona discharge to draw the liquid out into a regular pattern of diverging ligaments which break up into a cloud of charged atomised spray particles which is electrostatically attracted toward the plants.
 40. An electrostatic sprayer as in claim 39 wherein the spray orifice is of capillary dimensions.
 41. An electrostatic sprayer as in claim 39 or 40 wherein the potential applied to said nozzle is a single polarity potential. 